Photoactivation of aromatic molecules on the surface of magnesium oxide

Abstract

The influence of ultraviolet radiation on the state of molecules of aromatic hydrocarbons adsorbed from the gaseous phase onto magnesium oxide has been studied from the electronic absorption spectra (in diffuse reflected light) and electron paramagnetic resonance. Depending on the temperature of the preliminary vacuum treatment of the polycrystalline magnesium oxide, at least three types of molecular photoproducts were observed: paramagnetic products (for 600‡ C), nonparamagnetic products (for 400 and 900‡ C), and products identified as singly-charged radical-anions, converted into negative dimers in an excess of the hydrocarbon. A bathochromic shift of the absorption band maxima of the radical-anions is observed in the series BeO, MgO, CaO, and BaO. The mechanism of the photoformation of molecular ions and the nature of the surface defects on magnesium oxide are discussed.